Marine surveys conducted with multiple source arrays

ABSTRACT

Marine surveys carried out with multiple source arrays comprising three or more sources are discussed. Each source of a multiple source array is an array of source elements, such as air guns. The sources of a multiple source array may be arranged in particular type of configuration that is effectively maintained while the survey vessel travels a sail line. The sources of the multiple source array are activated to acoustically illuminate a subterranean formation with acoustic signals. Two or more sources of a multiple source array may be activated to create blended seismic data.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application62/246,283, filed Oct. 26, 2015.

BACKGROUND

Marine seismology companies invest heavily in the development of marineseismic surveying equipment and seismic data processing techniques inorder to obtain accurate, high-resolution seismic images of subterraneanformations located beneath a body of water. High-resolution seismicimages of a subterranean formation are used to determine the structureof subterranean formations, discover petroleum reservoirs, and monitorpetroleum reservoirs during production. A typical marine seismic surveyis carried out with a survey vessel that tows one or two seismic sourcesand a number of streamers through the body of water. The survey vesselcontains seismic acquisition equipment, such as navigation control,seismic source control, seismic receiver control, and recordingequipment. The seismic source control controls activation of the one ortwo seismic sources at selected times or locations. A seismic sourcetypically comprises an array of source elements, such as air guns, thatare simultaneously activated to produce an acoustic impulse. Theacoustic impulse is a sound wave that travels down through the water andinto a subterranean formation. At each interface between different typesof rock and sediment, a portion of the sound wave is refracted, aportion of the sound wave is transmitted, and another portion isreflected back into the body of water to propagate toward the watersurface. The streamers are elongated cable-like structures that aretowed behind the survey vessel in the direction the survey vessel istraveling (i.e., sail-line direction) and are arranged substantiallyparallel to one another in the direction perpendicular to the sail-linedirection. The streamers collectively form a seismic data acquisitionsurface. Each streamer includes a number of seismic receivers or sensorsthat detect pressure and/or particle motion wavefields of the soundwaves reflected back into the water from the subterranean formation. Therecorded pressure and/or particle motion wavefields are processed toproduce seismic images of the subterranean formation.

In order to reduce the cost per square kilometer of three-dimensionalseismic data acquisition and maximize the sub-surface area surveyed persail line, marine seismology companies often deploy longer streamerswith greater separation between the streamers as compared to moretraditional marine surveys. For example, a traditional marine seismicsurvey may be carried out with ten 6,000 m long streamers separated byabout 75 m By contrast, the acquisition time of a marine survey can belower with sixteen 7,000 m long streamers separated by 100 m, and stilllower with twelve 8,000 m long streamers separated by 150 m, and evenlower with ten 10,000 m long streamers separated by 200 m. In terms ofoverall marine survey productivity these large streamer separationsreduce data acquisition times from between 35% to 50%, which translatesinto a significant savings in time and costs.

However, efforts to lower cost by increasing streamer length anddistances between streamers may have a downside in that the spatialresolution of the seismic data collected is typically lower than theseismic data collected in surveys carried out with shorter, more closelyseparated streamers for the following reasons. As the lengths ofstreamers are increased, the seismic recording time intervals betweenseismic source activations is increased in order to capture returningsignals from longer source-receiver offsets. The longer recording timeintervals necessitate more time between activations of the seismicsources and, therefore, a larger sail-line distance is traveled betweenactivations of the seismic sources. In addition, as streamer separationincreases from 100 m, to 150 m, to 200 m, cross-line sampling betweenstreamers leads to spatial aliasing in the direction perpendicular tothe sail-line direction. As a result, traditional dual seismic sourceacquisition techniques combined with longer streamer lengths and largerdistances between streamers leads to a decrease in overall seismic datadensity, a decrease in the number of seismic source activations used toacoustically illuminate the subterranean formation, and increasedspatial aliasing perpendicular to the sail-line direction. These factorsimpact the ability to adequately sample reflected wavefields from thesubterranean formation, remove coherent noise, and provide sufficientspatial sampling to image complex subterranean geological structures.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show side-elevation and top views of an example seismic dataacquisition system.

FIGS. 2A-2B show side elevation and top views of a cross-line sourcearray.

FIGS. 3A-3B show side elevation and top views of a vertical-sourcearray.

FIGS. 4A-4C show side elevation, top, and rear views of a bent-sourcearray.

FIG. 5 shows an isometric view of a source.

FIG. 6 shows a side-elevation view of a seismic data acquisition systemwith a magnified view of a receiver.

FIG. 7 shows a full seismic data gather and example trace.

FIG. 8 shows a volume of seismic data in a space-frequency domain.

FIGS. 9A-9C show examples of two- and three-dimensional seismic datamatrices in a space-frequency domain.

FIG. 10A shows a top view of a typical marine survey carried out with asurvey vessel and two seismic sources space apart in the cross-linedirection.

FIG. 10B shows an elevation view of common midpoint and common depthpoints in three layers of a subterranean formation located beneath abody of water.

FIGS. 10C-10D shows top views of marine surveys carried out withcross-line source arrays.

FIGS. 11A-11C shows cross-line locations of a source and receiver usedto calculate cross-line particle velocity.

FIG. 12 shows the full seismic data gather in a wavenumber-frequencydomain.

FIGS. 13A-13C show noise attenuation using a two-dimensional mutingmask.

FIG. 14 shows a side-elevation view of a survey vessel that tows avertical-source array beneath a free surface of a body of water.

FIG. 15 shows a perspective view of a vertical-source array activated atfive locations along a sail line.

FIGS. 16A-16E shows source deghosting applied to acommon-receiver-station gather.

FIGS. 17A-17D show source deghosting applied to synthetic seismic data.

FIG. 18 shows a perspective view of a survey vessel that tows abent-source array beneath a free surface of a body of water.

FIG. 19 shows a perspective view of a bent-source array activated atthree locations along a sail line.

FIGS. 20A-20B show examples of recording blended seismic data as asurvey vessel travels a sail line of a marine seismic survey.

FIG. 21 shows a flow-control diagram of a method to deblend blendedseismic data.

FIG. 22 shows a flow-control diagram of a routine “estimate deblendedseismic data” called in FIG. 21.

FIGS. 23A-23C show source deblending applied to a common-shot gather ofpseudo-deblended seismic data produced with a source of avertical-source array.

FIGS. 24A-24D show deblended and unblended seismic data and differencebetween deblended and unblended seismic data shown in FIG. 23A-23C.

FIG. 25 shows example plots of source strength in a multiple sourcearray with increasing distance from a center of the multiple sourcearray.

FIG. 26 shows an example of a computer system that may be used toexecute efficient methods of attenuating noise, source deghosting, anddeblending seismic data according to embodiments disclosed herein.

DETAILED DESCRIPTION

Marine surveys carried out with multiple source arrays are described.Multiple source arrays may be used in marine surveys carried out withlonger streamers and greater separation between streamers than intraditional marine surveys in order to lower marine survey costs. Amultiple source array comprises three or more sources towed behind asurvey vessel. Each source of a multiple source array comprises an arrayof source elements. A source element may be an air gun, a water gun, ora marine vibrator. The sources of a multiple source array may bearranged and towed in a particular type of configuration. The sourcesmay be activated at that same time, according to a particular sequence,or at different times in order to acoustically illuminate a subterraneanformation with source wavefields that in turn produce reflectedwavefields from the subterranean formation. The sources of a multiplesource array may be activated so that the source wavefields overlap andthe reflected wavefields are blended and recorded as blended seismicdata. Methods to deblend, source deghost, and attenuate noise in blendedseismic data obtained using a multiple source array are described. Thesources of the multiple source arrays may be arranged in the cross-linedirection and spaced apart to maximize source wavefield interactionswith a subsurface area per sail line traveled by the survey vessel. Thesources of a multiple source array may be arranged vertically or alongdiagonals of a bent configuration and activated at different depths inorder to avoid generating source ghosts at the same frequency notchesfor an entire survey.

FIGS. 1A-1B show side-elevation and top views, respectively, of anexample seismic data acquisition system that includes a survey vessel102 towing a multiple source array 104 and six separate streamers106-111 beneath a free surface 112 of a body of water. The body of watercan be, for example, an ocean, a sea, a lake, or a river, or any portionthereof. In this example, each streamer is attached at one end to thesurvey vessel 102 via a streamer-data-transmission cable. Theillustrated streamers 106-111 form an ideally planar horizontal dataacquisition surface with respect to the free surface 112. However, inpractice, the data acquisition surface and streamers may be smoothlyvarying due to active sea currents and weather conditions. In otherwords, although the streamers 106-111 are illustrated in FIGS. 1A and 1Band subsequent figures as straight and substantially parallel to thefree surface 112, in practice, the towed streamers may undulate as aresult of dynamic conditions of the body of water in which the streamersare submerged. A data acquisition surface is not limited to having aplanar horizontal orientation with respect to the free surface 112. Thestreamers may be towed at depths that angle the data acquisition surfacewith respect to the free surface 112 or one or more of the streamers maybe towed at different depths. A data acquisition surface is not limitedto six streamers as shown in FIG. 1B. In practice, the number ofstreamers used to form a data acquisition surface can range from as fewas one streamer to as many as 20 or more streamers. The multiple sourcearray 104 comprises three or more sources described below with referenceto FIGS. 4-6.

FIG. 1A includes an xz-plane 114 and FIG. 1B includes an xy-plane 116 ofthe same Cartesian coordinate system having three orthogonal, spatialcoordinate axes labeled x, y and z. The coordinate system is used tospecify orientations and coordinate locations within the body of water.The x-direction specifies the position of a point in a directionparallel to the length of the streamers or in the direction the surveyvessel is traveling and is referred to as the “in-line” direction. They-direction specifies the position of a point in a directionperpendicular to the x-axis and substantially parallel to the freesurface 112 and is referred to as the “cross-line” direction. Thez-direction specifies the position of a point perpendicular to thexy-plane (i.e., perpendicular to the free surface 112) with the positivez-direction pointing downward away from the free surface 112. Thestreamers 106-111 are long cables containing power and data-transmissionlines that connect receivers represented by shaded rectangles, such asreceiver 118, spaced-apart along the length of each streamer to seismicacquisition equipment and data-storage devices located on board thesurvey vessel 102.

Streamer depth below the free surface 112 can be estimated at variouslocations along the streamers using depth-measuring devices attached tothe streamers. For example, the depth-measuring devices can measurehydrostatic pressure or utilize acoustic distance measurements. Thedepth-measuring devices can be integrated with depth controllers, suchas paravanes or water kites that control and maintain the depth andposition of the streamers as the streamers are towed through the body ofwater. The depth-measuring devices are typically placed at intervals(e.g., about 300 meter intervals in some implementations) along eachstreamer. Note that in other implementations buoys may be attached tothe streamers and used to maintain the orientation and depth of thestreamers below the free surface 112.

The multiple source array 104 may comprise two or more sources towedbehind the survey vessel 102 in a particular configuration. FIGS. 2A-2Bshow side elevation and top views, respectively, of the multiple sourcearray 104 in which the sources are towed behind the survey vessel 102 ina substantially linear configuration in the cross-line direction (i.e.,y-direction). The multiple source array 104 in which the sources arelinearly arranged in the cross-line direction is called a “cross-linesource array.” The sources may be spaced with approximately equaldistance between adjacent sources in the cross-line direction. FIGS.3A-3B show side elevation and top views, respectively, of the multiplesource array 104 in which the sources are towed behind the survey vessel102 in a substantially vertical direction (i.e., z-direction). Themultiple source array 104 in which the sources are substantiallylinearly arranged in the vertical direction is called a “vertical-sourcearray.” The sources may be spaced with approximately equal distancebetween adjacent sources in the vertical direction. The sources of themultiple source array 104 may also be towed behind the survey vessel 102as shown in FIGS. 4A-4C. The multiple source array 104 in which thesources are arranged along substantially linear diagonal directions asshown in FIGS. 4A-4C is called a “bent-source array.” The sources may bespaced with approximately equal distance between adjacent sources alongtwo downward directed diagonal lines with respect to the verticaldirection.

Each source of the multiple source array 104 comprises sub-arrays ofsource elements. The source elements may be air guns or water guns. FIG.5 shows an isometric view of an example source 500 that comprises foursimilarly configured sub-arrays 501-504. For example, the sub-array 503includes seven source elements, such as source element 506, suspendedfrom a semi-rigid rod 508 that is suspended from a float 510 by depthropes 512. The sub-array 503 also includes seven pressure sensors, suchas pressure sensor 514, that are each located in close proximity to oneof the source elements. For example, the pressure sensor 514 is locatedin the near field (e.g., approximately 1 m to less than 10 m) of thesource element 506. Point 516 represents the geometrical center of thesources of the source 500, where

_(s) represents the Cartesian coordinates (x_(s), y_(s), z_(s)) of thegeometrical center of the source elements of the source 500. Thesub-arrays 501-504 are connected by cables 518-520, and each sub-arraymay include a steering device, such as a wing, that may be used toseparately steer and control the direction the sub-array travels whilebeing towed through the body of water. For example, the sub-array 502includes a wing 522 that may be used to control the lateral direction ofthe sub-array 502.

Returning to FIG. 1A, FIG. 1A shows a cross-sectional view of the surveyvessel 102 towing the multiple source array 104 above a subterraneanformation 120. Curve 122, the formation surface, represents a topsurface of the subterranean formation 120 located at the bottom of thebody of water. The subterranean formation 120 may comprise a number ofsubterranean layers of sediment and rock. Curves 124, 126, and 128represent interfaces between subterranean layers of differentcompositions. A shaded region 130, bounded at the top by a curve 132 andat the bottom by a curve 134, represents a subterranean hydrocarbondeposit, the depth and positional coordinates of which may bedetermined, at least in part, by analysis of seismic data collectedduring a marine seismic survey. As the survey vessel 102 moves over thesubterranean formation 120, a seismic source control located on thesurvey vessel 102 activates the three or more sources comprising themultiple source array 104. The sources may be activated to produceblended acoustic signals at spatial and/or temporal intervals (seediscussion below with reference to FIG. 20). Activation of each sourceof the multiple source array 104 is often called a “shot.” In otherimplementations, the multiple source array 104 may be towed by onesurvey vessel and the streamers may be towed by a different surveyvessel. FIG. 1A shows an acoustic signal expanding outward from themultiple source array 104 as a pressure wavefield 136 represented bysemicircles of increasing radius centered at the multiple source array104. The outwardly expanding wavefronts from the multiple source arraymay be spherical but are shown in vertical plane cross section in FIG.1A. The outward and downward expanding portion of the pressure wavefield136 and any portion of the pressure wavefield 136 reflected from thefree-surface 112 are called the “source wavefield.” The source wavefieldeventually reaches the formation surface 122 of the subterraneanformation 120, at which point the source wavefield may be partiallyreflected from the formation surface 122 and partially refracteddownward into the subterranean formation 120, becoming elastic waveswithin the subterranean formation 120. In other words, in the body ofwater, the acoustic signal is composed primarily of compressionalpressure waves, or P-waves, while in the subterranean formation 120, thewaves include both P-waves and transverse waves, or S-waves. Within thesubterranean formation 120, at each interface between different types ofmaterials or at discontinuities in density or in one or more of variousother physical characteristics or parameters, downward propagating wavesmay be partially reflected and partially refracted. As a result, eachpoint of the formation surface 122 and each point of the interfaces 124,126, and 128 may be a reflector that becomes a potential secondary pointsource from which acoustic and elastic wave energy, respectively, mayemanate upward toward the receivers 118 in response to the acousticsignal generated by the multiple source array 104 anddownward-propagating elastic waves generated from the pressure impulse.As shown in FIG. 1A, waves of significant amplitude may be generallyreflected from points on or close to the formation surface 122, such aspoint 138, and from points on or very close to interfaces in thesubterranean formation 120, such as points 140 and 142. The upwardexpanding waves reflected from the subterranean formation 120 arecollectively the “reflected wavefield.”

The waves that compose the reflected wavefield may be generallyreflected at different times within a range of times following theinitial source wavefield. A point on the formation surface 122, such asthe point 138, may receive a pressure disturbance from the sourcewavefield more quickly than a point within the subterranean formation120, such as points 140 and 142. Similarly, a point on the formationsurface 122 directly beneath the multiple source array 104 may receivethe pressure disturbance sooner than a more distant-lying point on theformation surface 122. Thus, the times at which waves are reflected fromvarious points within the subterranean formation 120 may be related tothe distance, in three-dimensional space, of the points from theactivated multiple source array 104.

Acoustic and elastic waves, however, may travel at different velocitieswithin different materials as well as within the same material underdifferent pressures. Therefore, the travel times of the source wavefieldand reflected wavefield may be functions of distance from the multiplesource array 104 as well as the materials and physical characteristicsof the materials through which the wavefields travel. In addition,expanding wavefronts of the wavefields may be altered as the wavefrontscross interfaces and as the velocity of sound varies in the mediatraversed by the wavefront. The superposition of waves reflected fromwithin the subterranean formation 120 in response to the sourcewavefield may be a generally complicated wavefield that includesinformation about the shapes, sizes, and material characteristics of thesubterranean formation 120, including information about the shapes,sizes, and locations of the various reflecting features within thesubterranean formation 120 of interest to exploration seismologists.

Each receiver 118 may be a multi-component sensor including particlemotion sensors and a pressure sensor. A pressure sensor detectsvariations in water pressure over time. The term “particle motionsensor” is a general term used to refer to a sensor that may beconfigured to detect particle displacement, particle velocity, orparticle acceleration over time.

FIG. 6 shows a side-elevation view of the marine seismic dataacquisition system with a magnified view 602 of the receiver 118. Thereceiver 118 is a multi-component sensor that comprises a pressuresensor 604 and a particle motion sensor 606. The pressure sensor may be,for example, a hydrophone. Each pressure sensor is a non-directionalsensor that measures changes in hydrostatic pressure over time toproduce pressure data denoted by p(

_(r),

_(s), t), where represents the Cartesian coordinates (x_(r), y_(r),z_(r)) of a receiver, and t represents time. The particle motion sensorsmay be responsive to water motion. In general, particle motion sensorsdetect particle motion (i.e., displacement, velocity, or acceleration)in a particular direction and may be responsive to such directionaldisplacement of the particles, velocity of the particles, oracceleration of the particles. A particle motion sensor that measuresparticle displacement generates particle displacement data denoted by

(

_(r),

_(s), t), where the vector

represents the direction along which particle displacement is measured.A particle motion sensor that measures particle velocity (i.e., particlevelocity sensor) generates particle velocity data denoted by

(

_(r),

_(s), t). A particle motion sensor that measures particle acceleration(i.e., accelerometer) generates particle acceleration data denoted by

(

_(r),

_(s), t). The data generated by one type of particle motion sensor maybe converted to another type. For example, particle displacement datamay be differentiated to obtain particle velocity data, and particleacceleration data may be integrated to obtain particle velocity data.

The term “particle motion data” is a general term that refers toparticle displacement data, particle velocity data, or particleacceleration data, and the term “seismic data” refers to pressure dataand/or particle motion data. The pressure data represents a pressurewavefield, particle displacement data represents a particle displacementwavefield, particle velocity data represents a particle velocitywavefield, and particle acceleration data represents particleacceleration wavefield. The particle displacement, velocity, andacceleration wavefields are referred to as particle motion wavefields.

The particle motion sensors are typically oriented so that the particlemotion is measured in the vertical direction (i.e.,

=(0, 0, z)) in which case g_(z)(

_(r),

_(s), t) is called vertical displacement data, v_(z)(

_(r),

_(s), t) is called vertical velocity data, and a_(z)(

_(r),

_(s), t) is called vertical acceleration data. Alternatively, eachreceiver may include two additional particle motion sensors that measureparticle motion in two other directions,

₁ and

₂, that are orthogonal to

(i.e.,

⋅

n₁=

⋅

₂=0, where “⋅” is the scalar product) and orthogonal to one another(i.e.,

₁⋅

₂=0). In other words, each receiver may include three particle motionsensors that measure particle motion in three orthogonal directions. Forexample, in addition to having a particle motion sensor that measuresparticle velocity in the z-direction to give v_(z)(

_(r),

_(s), t), each receiver may include a particle motion sensor thatmeasures the wavefield in the in-line direction in order to obtain theinline velocity data, v_(x)(

_(r),

_(s), t), and a particle motion sensor that measures the wavefield inthe cross-line direction in order to obtain the cross-line velocitydata, v_(y)(

_(r),

_(s), t). In certain implementations, the receivers may comprise onlypressure sensors, and in other implementations, the receivers may c onlyparticle motion sensors.

The streamers 106-111 and the survey vessel 102 may include sensingelectronics and data-processing facilities that allow seismic datagenerated by each receiver to be correlated with the time each source ofthe multiple source array 104 is activated, absolute positions on thefree surface 112, and absolute three-dimensional positions with respectto an arbitrary three-dimensional coordinate system. The pressure dataand particle motion data may be stored at the receiver, and/or may besent along the streamers via data transmission cables to the surveyvessel 102, where the data may be stored electronically or magneticallyon data-storage devices located onboard the survey vessel 102 and/ortransmitted onshore to a seismic data-processing facility.

Each pressure sensor and particle motion sensor, or each group of suchsensors, may include an analog-to-digital converter that convertstime-dependent analog signals into discrete time series of consecutivelymeasured values called “amplitudes” separated in time by a sample rate.The time series data generated by a pressure or particle motion sensoris called a “trace,” which may comprise thousands of samples collectedat a typical sample rate of about 1 to 5 ms. A trace is a recording of asubterranean formation response to acoustic energy that passes from anactivated source, into the subterranean formation where a portion of theacoustic energy is reflected and/or refracted, and ultimately detectedby a sensor as described above. A trace records variations in atime-dependent amplitude that corresponds to fluctuations in acousticenergy of the wavefield measured by the sensor. In general, each traceis an ordered set of discrete spatial and time-dependent pressure ormotion sensor amplitudes denoted by:tr(

_(r),

_(s) ,t)={A(

_(r),

_(s) ,t _(j))}_(j=0) ^(J−1)  (1)

where

tr represents pressure, particle displacement, particle velocity, orparticle acceleration amplitude;

A is amplitude;

t_(j) is the jth sample time; and

J is the number of time samples in the trace.

The coordinate location

_(r) of each receiver may be calculated from global position informationobtained from one or more global positioning devices located along thestreamers, survey vessel, and buoys and the known geometry andarrangement of the streamers and receivers. The coordinate location

_(s) of each source within the multiple source array 104 may also beobtained from one or more global positioning devices located at eachsource and the known geometry and arrangement of the sources within themultiple source array 104. Each trace may also include a trace headernot represented in Equation (1) that identifies the specific receiverthat generated the trace, receiver and source GPS spatial coordinates,and may include time sample rate and the number of time samples. Thetraces may be recorded in recording equipment located on board thesurvey vessel 102.

The traces are collected to form a “gather” that can be furtherprocessed using various seismic data processing techniques in order toobtain information about the structure of the subterranean formation.FIG. 7 shows a seismic data set 700 collected for a number of differentsource and receiver coordinate locations. Axis 701 represents sourcecoordinates, axis 702 represents receiver coordinates, and axis 703represents time. A trace 704 of seismic data p(

_(r),

_(s), t), as described above with reference to Equation (1), for anacoustic signal generated by a source at a source location

_(s) of a multiple source array and measured by a receiver, such as apressure sensor or particle motion sensor, at a receiver location

_(r). Note that the wiggle in the trace 704 does not representtime-dependent variation with respect to receiver and sourcecoordinates. The wiggle in the trace 704 represents time-dependentvariation in the pressure by a pressure sensor located at the sourcelocation

_(s) and the receiver location x_(r). Traces of seismic data not shownin FIG. 7 are recorded for other source and receiver locationsrepresented by source and receiver axes 701 and 702. The traces arecollected to form the seismic data set 700:p (

_(r),

_(s) ,t)={p(

_(r),

_(s) ,t)}  (2)The traces in the seismic data set 700 may be sorted into differentdomains, such as the common-shot domain, common-receiver-station domain,common-channel domain, and common-midpoint domain. For example,rectangle 706 represents a common-shot gather of traces having the samesource location and different receiver locations, such as tracescollected along the same streamer. Rectangle 708 represents acommon-receiver-station gather of traces having different sourcelocations but the trace are generated by different receivers that passover the same or common-receiver-station location.

Note that in the following discussion the general term “seismic data” isused to refer to pressure data, particle motion data, particle velocitydata, and particle acceleration data. The notation, p, is used torepresent pressure data, particle motion data, particle velocity data,or particle acceleration data in mathematical formulas discussed below,unless stated otherwise. The methods and systems discussed below are notintended to be limited to pressure data and may be used with particlemotion data, particle velocity data, and particle acceleration datadescribed above.

A Fourier transform may be used to transform the seismic data set 700from the space-time (“s-t”) domain to the space-frequency (“s-f”) domainas follows:

_(t→f)( p (

_(r),

_(s) ,t)= P (

_(r),

_(s) ,f)  (3)

where

_(t-f) is a time-to-frequency domain Fourier transform; and

P is seismic data set in the s-f domain.

FIG. 8 shows a seismic data set 800, denoted by P, in the s-f domainproduced by applying a Fourier transform with respect to time to theseismic data set 700. Axis 801 represents frequencies denoted by f. Thedashed line layers in the seismic data set 800 represent seismic datamatrices for different frequencies. For example, shaded layer 802represents a seismic data matrix denoted by P(

_(r),

_(s), f) for a single frequency f. The seismic data set P has a seismicdata matrix P for each frequency.

FIG. 9A shows a two-dimensional seismic data matrix 900 for a singlefrequency of a seismic data set in the s-f domain. Vertical axis 901represents receiver locations, and horizontal axis 902 represents sourcelocations. In this example, N represents the number of sources of amultiple source array and M represents the number of receivers. Each X,such as X 903, within the seismic data matrix 900 represents seismicdata generated by a receiver at a receiver location as a result ofactivating a source of the multiple source array at a source location.

FIG. 9B shows an example of a three-dimensional seismic data matrix 908for a single frequency of a seismic data set in the s-f domain generatedby a cross-line source array. Directional arrows 910 and 912 representin-line source and receiver coordinates, respectively. Each squarerepresents a submatrix of seismic data associated with a particular pairof in-line source and receiver coordinates and the ranges of cross-linereceiver and source coordinates. For example, directional arrows 914 and916 represent cross-line source and receiver coordinates, respectively.Submatrix 918 represents seismic data generated by a source at in-linereceiver coordinate x_(r1) and in-line source coordinate x_(s2) for arange of cross-line coordinate receiver and source coordinates y_(r) andy_(s). Dashed line 920 represent a three-dimensionalcommon-receiver-station gather, and dashed line 922 represents athree-dimensional common-shot gather. On the other hand,three-dimensional common channel gather and common midpoint gather arebroken up over the submatrices.

FIG. 9C shows an example of a three-dimensional seismic data matrix 926for a single frequency of a seismic data set in the s-f domain generatedby a vertical-source array. Directional arrows 928 and 930 representin-line source and receiver coordinates, respectively. Each squarerepresents a submatrix of seismic data associated with a particular pairof in-line source and receiver coordinates and ranges of cross-linereceiver and vertical source coordinates. For example, directionalarrows 932 and 934 represent cross-line receiver and vertical sourcecoordinates, respectively. Submatrix 936 represents a seismic datagenerated by a source at in-line receiver coordinate x_(r1) and in-linesource coordinate x_(s2) for a range cross-line coordinate receivercoordinates y_(r) and vertical source coordinates z_(s). Dashed line 938represent a three-dimensional common-receiver-station gather, and dashedline 940 represents a three-dimensional common-shot gather. On the otherhand, three-dimensional common channel gather and common midpoint gatherare broken up over the submatrices.

The seismic data in the seismic data matrix may be sorted into differentdomains determined by the source and receiver coordinates. Differentdomains include a common-shot domain, common-channel domain,common-receiver-station domain, and common-midpoint (“CMP”) domain. FIG.9A shows different ways seismic data of the seismic data matrix 900 maybe sorted into different domains. A dashed line 904 represents acommon-shot gather formed from the seismic data generated by thereceivers and one of the sources of the multiple source array activatedat a source location. A dashed line 905 represents acommon-receiver-station gather formed from the seismic data generated byreceivers that pass over the same receiver-coordinate location for thesources of the multiple source array activated at the N different sourcelocations. Diagonal dashed line 906 represents a common-channel gatherof seismic data. Seismic data generated by receivers at the receiverlocations and source location along a diagonal represented by dashedline 907 are collected to form a common-midpoint (“CMP”) gather. The CMPis the point on the formation surface halfway between the location ofthe source and the location of the receiver. The CMP is not to beconfused with common depth points that are located below the CMP atinterfaces within the subterranean formation.

As the number of sources in a cross-line source array increases andseparation distance between the sources decreases, the density of sourcewavefield interactions with a subterranean formation is increased. Theconcept of increasing the density of source wavefield interactions asthe number of sources increases and separation distances between sourcesof a cross-line source array decreased is represented in FIGS. 10A-10Dwith CMPs. FIG. 10A shows a top view of a typical marine survey carriedout with a survey vessel 1001 that tows two seismic sources 1002 and1003 and a number of streamers of which only four streamers are shown.The streamers are separated by a distance D in the cross-line direction.Shaded region 1004 represents a region of the surface of a subterraneanformation and dots represent CMPs. FIG. 10B shows a cross-sectional viewof three different horizontal subterranean layers 1007-1009, representedby different shadings, of a subterranean formation located beneath abody of water 1010. A dot 1011 represents a CMP between source locations1012 and receiver locations 1013. The CMP 1011 is located above commondepth points, which corresponds to reflection points between the layers1007-1009 along dot-dashed line 1014. The density of source wavefieldinteractions with a subterranean formation (i.e., increase the number ofCMPs) may be increased by increasing the number of sources in across-line source array having a total overall length in the cross-linedirection that is less than the distance D between streamers, as shownin FIGS. 10C and 10D. In FIG. 10C, a survey vessel 1016 tows across-line source array 1018 of four sources separated in the cross-linedirection by about one quarter of the distance D (i.e., D/4) separatingthe streamers. The total length of the cross-line source array 1018 isabout 3D/4. As a result, the CMPs are separated in the cross-linedirection by about one eighth of the distance D (i.e., D/8) separatingthe streamers. In FIG. 10D, a survey vessel 1020 tows a cross-linesource array 1022 of eight sources separated in the cross-line directionby about one eighth of the distance D (i.e., D/8) separating thestreamers in the cross-line direction. The total length of thecross-line source array 1022 is about 7D/8. As a result, the CMPs areseparated in the cross-line direction by about one sixteenth of thedistance D (i.e., D/16) separating the streamers in the cross-linedirection. In general, a cross-line source array of N sources has atotal length in the cross-line direction less than D and the distancebetween each pair of adjacent sources is about DIN, which gives CMPsseparated by D/2N in the cross-line direction.

Using cross-line source arrays that comprise three or more sources witha total cross-line source array length that is less than the distancesseparating the streamers increases the density of source wavefieldinteractions with the subterranean formation, which allows for a largercross-line and in-line spacing between receivers located along thestreamers. As a result, a larger volume space of a subterraneanformation may be sampled by source wavefields per sail line, whichresults in a reduction in marine survey costs.

Cross-line source arrays and bent-source-array also enable calculationof particle velocity data in the cross-line direction at receiverlocations by assuming source location reciprocity and using pressuredata recorded at a receiver location. FIGS. 11A-11C show calculation ofcross-line particle velocity at a receiver location 1101. In FIG. 11A,three different sources 1102-1104 of a cross-line, or bent, source array1106 are activated at cross-line coordinate locations (x_(s), y_(i)),(x_(s), x_(j)), and (x_(s), y_(k)). Note that the z-coordinate in thefollowing discussion is suppressed because the three sources 1102-1104may belong to either a cross-line source array or a bent-source array.In this example, the quantities P(

_(r), x_(s), y_(i), f), P(

_(r), x_(s), y_(k), f), and P(

_(r), x_(s), y_(k), f) represent pressure data measured at the samereceiver location

for separate activations of the three sources 1102-1104. Therelationship between the gradient of the pressure and the particlevelocity in the general particle displacement direction

is given by

$\begin{matrix}{V_{\overset{\rightharpoonup}{n}} = {{- \frac{i}{\rho\omega}}{\overset{\rightharpoonup}{n} \cdot {\overset{\rightharpoonup}{\nabla}P}}}} & (4)\end{matrix}$

where

i is the imaginary unit √{square root over (−1)};

ρ is the density of water;

ω w is the angular frequency (i.e., ω=2πf); and

P is the pressure gradient.

Setting the particle displacement direction

=(0,1,0) in Equation (4) (i.e., cross-line particle displacementdirection) gives the cross-line velocity:

$\begin{matrix}{V_{y} = {{- \frac{i}{\rho\omega}}\frac{\partial P}{\partial y}}} & (5)\end{matrix}$

where ∂P/∂V is the pressure gradient in the cross-line direction.

An approximate cross-line particle velocity at a point between thesources 1102 and 1104 may be calculated by

$\begin{matrix}{{V_{y} \approx {{- \frac{i}{\rho\omega}}\frac{\Delta\; P}{\Delta\; y}}} = {{- \frac{i}{\rho\omega}}\frac{{P\left( {{\overset{\rightharpoonup}{x}}_{r},x_{s},y_{k},f} \right)} - {P\left( {{\overset{\rightharpoonup}{x}}_{r},x_{s},y_{i},f} \right)}}{y_{k} - y_{i}}}} & (6)\end{matrix}$The differential Δy in the denominator of Equation (6) is calculated asthe distance 1108 between the sources 1102 and 1104, as shown in FIG.11B, and the differential ΔP in the numerator of Equation (5) is givenby the difference between pressure data generated by receiver 1101 inresponse to activation of the sources 1102 and 1104. Equation (6) givesan approximate cross-line particle velocity at the location y_(j) of thesource 1103 and is represented by a vector 1110 (i.e., (0, V_(y), 0)).In the example of FIG. 11, the location of the source 1103 represents apoint between the sources 1102 and 1104. Note that the source 1103 doesnot have to be present in order to calculate the approximate cross-lineparticle velocity according to Equation (6) at a point between thesources 1102 and 1104. There may be zero or any number of other sourcesbetween the sources 1102 and 1104. Source location reciprocity is usedto obtain an approximate cross-line particle velocity (0, −V_(y), 0) atthe receiver location 1101 represented by a vector 1112 in FIG. 11C. Asstreamer separation is increased, the cross-line particle velocity (0,−V_(y), 0) may be used to interpolate cross-line particle velocitiesbetween adjacent streamers and thereby reduce cross-line spatialaliasing.

Seismic data is often contaminated with noise represented byp(

_(r),

_(s) ,t)=p _(des)(

_(r),

_(s) ,t)+n(

_(r),

_(s) ,t)  (7)

where

p_(des)(

_(r),

_(s), t) is a signal component of the seismic data; and

n(

_(r),

_(s), t) is noise.

The signal component of the seismic data is typically coherent while thenoise is incoherent. The noise may be the result of acoustic andnon-acoustic energy, such as propeller noise, noise due to free surfacewaves, noise due to streamer vibrations typically caused by devicesattached to the streamer, noise due to any irregularities on thestreamer surface (e.g., barnacles), and other noise sources.

The frequency f of a sound wave, wavenumber k of the sound wave, andspeed c of sound wave propagating in water are related by f=kc. Becausethe signal component of the seismic data propagates along the streamerat apparent speeds greater than or equal to c, the signal component lieswithin a signal region defined by frequency-to-wavenumber ratios greaterthan or equal to c (i.e., f/k>c). The signal region contains amplitudesassociated with energy that propagates at speeds greater than or equalto c. The signal region will also contain any noise components thatpropagate at speeds greater than or equal to c. In other words, thenoise n(

_(r),

_(s), t) may be decomposed into low-speed noise and high-speed noisecomponents as follows:n(

_(r),

_(s) ,t)=n _(<c)(

_(r),

_(s) ,t)+n _(≥c)(

_(r),

_(s) ,t)  (8)

where

n_(<c)(

_(r),

_(s), t) is low-speed noise that propagates at speeds less c; and

n_(≥c)(

_(r),

_(s), t) is high-speed noise that propagates at speeds greater than c.

The signal region may be determined by transforming the seismic data setor a gather of seismic, shown in FIG. 7, from the s-t domain to awavenumber-frequency (“w-f”) domain. FIG. 12 shows seismic data, such asthe seismic data set 700 or a gather of the seismic data set 700,transformed 1201 from the s-t domain to the w-f domain. In the w-fdomain, axis 1202 represents in-line wavenumbers (i.e., k_(x)) and axis1204 represents cross-line wavenumbers (i.e., k_(y)). Axis 1206represents frequencies. A cone 1208 represents a signal region. Theboundary of the signal region 1208 is given by

$\begin{matrix}{c = \frac{f}{\sqrt{k_{x}^{2} + k_{y}^{2}}}} & (9)\end{matrix}$Points outside the signal region, such as point (f₁, k_(x1), k_(y1))1210, have speeds that are greater than the speed of sound in water cand satisfy the following condition:

$\begin{matrix}{c < \frac{f_{1}}{\sqrt{\left( k_{x\; 1} \right)^{2} + \left( k_{y\; 1} \right)^{2}}}} & (10)\end{matrix}$Points inside the signal region, such as point (f₂, k_(x2), k_(y2))1212, have speeds that are less than or equal to the speed of sound inwater c and satisfy the following condition:

$\begin{matrix}{\frac{f_{2}}{\sqrt{\left( k_{x\; 2} \right)^{2} + \left( k_{y\; 2} \right)^{2}}} \leq c} & (11)\end{matrix}$Points located inside the signal region 1208 correspond to the signalcomponent of the seismic data and any noise that propagates at speedsless than or equal to the speed of sound in water c. While pointslocated outside the signal region 1208 correspond to noise thatpropagates at speeds greater than then speed of sound in water.

High speed noise in the seismic data may be attenuated by applying a 2Dmuting mask, such as the example w-f muting mask shown in FIG. 13A, foreach frequency in the w-f domain. In FIG. 13A, the two-dimensional(“2D”) muting mask contains a lightly shaded circular region 1301surrounded by a darker shaded region 1302. The radius 1303 of thecircular region is the speed of sound in water c. Points that lie withina plane defined by the 2D muting mask have the same frequency f butdifferent in-line and cross-line wavenumbers. Points that lie within thecircular region 1301 satisfy the condition given by Equation (11), andpoints that lie outside the circular region 1301 satisfy the conditionin Equation (10). The 2D muting mask may be used to remove high speednoise from the seismic data by multiplying points in the w-f frequencydomain that lie within the circular region 1301 by “1” and multiplyingpoints in the w-f frequency domain that lie outside the circular region1301 by “0.” In other words, for each frequency f, if associated in-lineand cross-line wavenumbers satisfy the condition

$\begin{matrix}{\frac{f}{c} \leq \sqrt{k_{x}^{2} + k_{y}^{2}}} & (12)\end{matrix}$the point is unchanged. On the other hand, for each frequency f, ifassociated in-line and cross-line wavenumbers satisfy the condition

$\begin{matrix}{\sqrt{k_{x}^{2} + k_{y}^{2}} < \frac{f}{c}} & (13)\end{matrix}$the point is muted.

FIG. 13B shows a snapshot of three-dimension filtering applied in thew-f domain using a 2D muting filter. Points 1304 and 1306 have the samefrequency have different in-line and cross-line wavenumbers. The point1304 satisfies Equation (12) and is retained, but the point 1306satisfies Equation (13) and is muted. Muting in this manner is repeatedfor each frequency as indicated by directional arrow 1308, leaving noiseattenuated seismic data that may still contain low speed noise.

The seismic data in the signal region 1208 may be transformed back tothe s-t domain to obtain noise attenuated seismic data set 1310 shown inFIG. 13C. A trace 1312 corresponds to a trace 704 in the seismic dataset 700 includes the signal component of the seismic data and any lowspeed noise that falls within the signal region 1208 as follows:{circumflex over (p)}(

_(r),

_(s) ,t)=p _(des)(

_(r),

_(s) ,t)+n _(<c)(

_(r),

_(s) ,t)  (14)Noise attenuation may be applied to two- and three-dimensionalcommon-shot gathers or two- and three-dimensionalcommon-receiver-station gathers and depends on the sampling in thein-line and cross-line directions.

Marine surveys carried out using a vertical-source array produce seismicdata that may be source deghosted in the w-f domain. FIG. 14 shows aside elevation view of a survey vessel 1401 that tows a vertical-sourcearray 1402 beneath the free surface of a body of water. Each source inthe vertical-source array 1402 has a corresponding ghost source locatedsubstantially the same distance above the free surface. For example, thesource 1406 in the vertical-source array 1404 is located at a depth D′below the free surface. The source 1406 has a corresponding ghost sourcelocated substantially the same distance D′ above the free surface. Whenthe source 1406 is activated, a source wavefield expands outward fromthe source 1406 in all directions. The free surface operates as a nearlyperfect acoustic reflector by downwardly reflecting the portion of thesource wavefield that propagates upward from the source 1406. Thedownward reflected portion of the source wavefield appears to have beengenerated by a virtual or ghost source located above the free surfaceand activated at approximately the same time as the correspondingsource. For example, directional arrows 1410 and 1412 representpropagation directions of two different portions of the same sourcewavefield generated by the source 1406. The first portion propagatesdownward in the direction 1410. The second portion propagates upward inthe direction 1412 and is reflected downward from the free surface in adirection represented by directional arrow 1414. The downwardlyreflected source wavefield trails behind the downward source wavefieldas if the downwardly reflected source wavefield had been generated bythe ghost source 1408 located above the free surface, as indicated by adashed-line extension 1416 of the directional arrow 1414.

The ghost sources effectively doubles the number of sources in thevertical-source array. For example, if a vertical-source array comprisesten sources, then the vertical-source array effectively operates as avertical-source array comprising twenty sources. However, the ghostsources were not actually activated and the source wavefields thatappear to have been generated by the ghost sources trail behind thesource wavefields generated by the sources, which creates ghost effectsor noise that adversely effects any resulting seismic images.

Source deghosting may be performed on seismic data in a marine surveyconducted with a vertical-source array by first sorting the seismic datainto common-receiver-station gathers for each separate activation of thesources of the vertical-source array. FIG. 15 shows a side-elevationview of a vertical-source array comprising six sources activated at fivedifferent locations along a sail line 1502. The survey vessel that towsthe vertical-source array is not shown. Box 1504 represents acommon-receiver-station location in which source and reflectedwavefields are measured by one or more receivers located at thecommon-receiver-station location for activations of the vertical-sourcearray. Shaded rectangles represent common-receiver-station gathersformed for each separate activation of the sources of thevertical-source array along the sail line 1502. For example,common-receiver-station gather 1506 comprises traces generated byreceivers that have common-receiver-station location 1504 when the sixsources 1508 of the vertical-source array are activated at sail linelocation 1510 The common-receiver-station gather 1506 is a seismic datarecord of the source and reflected wavefields that result fromactivation of the sources 1508 of the vertical-source array and is arecord of ghost source and ghost reflected wavefields associated withghost sources 1512. Common-receiver-station gathers 1514-1517 of tracesgenerated by receivers that are located at common-receiver-stationlocation 1504 are formed for each separate activation of thevertical-source array at corresponding points 1518-1521 along the sailline. The common-receiver-station gathers 1506 and 1514-1517 are eachtwo-dimensional gathers. The common-receiver-station gathers 1506 and1514-1517 may be combined with the source coordinates 1510 and 1518-1521to form a three-dimensional common-receiver-station gather as describedabove with reference to FIG. 9C.

FIGS. 16A-16E shows source deghosting a common-receiver-station gather1602. FIG. 16A shows the common-receiver-station gather. Vertical axis1604 represents time. Horizontal axis 1606 represents source depth withpositive values corresponding to depths of sources of thevertical-source array and negative values corresponding to distances ofghost sources above the free surface. The traces with source depths inthe interval [+D, 0] are associated with actual sources located belowthe free surface. By contrast, traces with source depths in the interval[0, −D] are associated with ghost sources. The traces with source depthsin the interval [0, −D] may be obtained by copying each trace withsource depths in the interval [+D, 0]. In the case of pressure data, theamplitudes in the trace may be multiplied by −1 in order to incorporatefree surface reflectivity. Negative sloped line 1608 representsreflection events from a subterranean surface produced by sourcewavefields associated with sources of the vertical-source array.Positively sloped line 1610 represents reflection events from thesubterranean surface produced by source wavefields associated withghosts of the vertical-source array. The common-receiver-station gatheris transformed from the s-t domain to the w-f domain shown in FIG. 16B.Vertical axis 1612 represents frequency. Horizontal axis 1614 representswavenumbers in the z-direction. Shaded region 1618 represents a signalregion that contains signal components of the seismic data in thecommon-receiver-station gather. Dashed-dot line 1620 divides the signalregion into a first signal region 1622 and a second signal region 1624.The first signal region 1622 corresponds to negative z-directionwavenumbers and encompasses signal components associated with sources ofthe vertical-source array. The second signal region 1624 corresponds topositive z-direction wavenumbers and encompasses signal componentsassociated with the ghost sources of the vertical-source array. In FIG.16C, muting is applied to seismic data with corresponding z-directionwavenumbers greater than zero. After muting, the signal components inthe first signal region 1622 remains, as shown in FIG. 16D. The seismicdata remaining after muting is transformed from the w-f domain to thes-t to obtain a source-deghosted common-receiver-station gather shown inFIG. 16E. Notice that only the reflection events 1608 associated withthe sources of the vertical-source array remain. The reflection events1610 associated with the source ghosts have been removed.

FIGS. 17A-17D show source deghosting applied to synthetic seismic data.FIG. 17A shows a synthetic common-receiver-station gather of seismicdata generated by a vertical-source array, containing 12 real and 12ghosts. The gather includes crossing reflection events 1702 and 1704.Negative slope reflection events 1702 result from source wavefieldsproduced by sources of the vertical-source array. Positive slopereflection events 1704 result from source wavefields produced by ghostsof the vertical-source array. FIG. 17B shows the synthetic seismic dataof the common-receiver-station gather in FIG. 17A transformed to the w-fdomain. Shade regions 1706 and 1708 represents the reflection events1702 and 1704, respectively, in the w-f domain. FIG. 17C shows thesynthetic seismic data in the w-f domain after muting is applied toseismic data with z-direction wavenumbers greater than zero. FIG. 17Dshows a source-deghosted common-receiver-station gather obtained bytransforming the seismic data in FIG. 17C to the s-t domain. Notice thatthe reflection events 1704 associated with the ghosts of thevertical-source array have been removed.

For marine surveys conducted with a bent-source array, source deghostingmay also be carried out in the w-f domain as described above withreference to FIGS. 16 and 17. FIG. 18 shows a perspective view of asurvey vessel 1801 that tows a bent-source array 1802 beneath the freesurface of a body of water. Like with the vertical-source array, eachsource in the bent-source array 1802 has a corresponding ghost sourcelocated above the free surface. For example, when the source 1806 isactivated, a source wavefield expands outward from the source 1806 inall directions. The portion of the source wavefield that is incident onthe free surface is reflected downward as if the downward reflectedportion of the source wavefield had been generated by a ghost source1808 located above the free surface. Directional arrows 1810 and 1812represent propagation directions of two different portions of the samesource wavefield generated by the source 1806. The first portionpropagates in the downward direction 1810. The second portion propagatesin an upward direction 1812 and is downwardly reflected from the freesurface in a direction represented by directional arrow 1814. Thedownwardly reflected source wavefield trails behind the downward sourcewavefield as if the downwardly reflected source wavefield had beengenerated by the ghost source 1808 located above the free surface, asindicated by a dashed-line extension 1816 of the directional arrow 1814.

The ghost sources effectively double the number of sources in thebent-source array. For example, the bent-source array 1802 comprises sixsources. Reflections from free surface cause the bent-source array 1802to effectively operate as a bent-source array with twelve sources.However, the ghost wavefields that appear to have been generated by theghost sources trail behind the source wavefields generated by the actualsources, which creates ghost affects or noise that adversely effects anyresulting seismic images.

Source deghosting may be performed on seismic data produced for a surveyconducted with a bent-source array in the same manner that sourcedeghosting is described above with reference to FIG. 16. The seismicdata for each activation of bent-source array 1802 is sorted intocommon-receiver-station gathers. FIG. 19 shows a perspective view of abent-source array configured with six sources activated at threedifferent locations along a sail line 1902. The survey vessel that towsthe bent-source array is not shown. Box 1904 represents acommon-receiver-station location in which source and reflectedwavefields are measured by one or more receivers that are located at orpass over the common-receiver-station location for each activation ofthe bent-source array. Shaded rectangles representcommon-receiver-station gathers formed for each activation of thebent-source array along the sail line 1902. For example,common-receiver-station gather 1906 comprises traces generated by one ormore receivers located at common-receiver-station location 1904 when thebent-source array is activated at sail line location 1908. Thecommon-receiver-station gather 1906 is a seismic data record of thesource and reflected wavefields that result from activation of thesources 1910 of the bent-source array and is a record of ghost sourceand reflected wavefields associated with ghost sources 1912.Common-receiver-station gathers 1914 and 1915 include traces generatedby one or more receivers that are located at or pass over thecommon-receiver-station location 1904 and are formed for each separateactivation of the bent-source array at corresponding points 1918-1919along the sail line. The common-receiver-station gathers 1906, 1914, and1915 are two-dimensional gathers that may be combined with the sourcecoordinates 1919, 1918, and 1919 to form a three-dimensionalcommon-receiver-station gather.

Each of the common-receiver-station gathers formed for each activationof the bent-source array in FIG. 19 may be source deghosted in the samemanner as the common-receiver-station gathers produced for eachactivation of the vertical-source array described above with referenceto FIG. 16.

In practice, the sources of a multiple source array may be activated tocreate blended seismic data. FIG. 20A shows an example of recordingblended seismic data in separate shot records as a survey vessel travelsa sail line of a marine seismic survey. In the example of FIG. 20A, asurvey vessel 2002 tows six streamers 2004 and a multiple source array2006 along a sail line 2008. The multiple source array 2006 may be across-line source array, a vertical-source array, a bent-source array,or another arrangement of sources. In other embodiments, more or fewerstreamer may be used. FIG. 20 includes a time axis 2010 with start timesof three activation-time intervals 2012-2014 denoted by t₀, t₁, and t₂.The sources of the multiple source array 2006 are activated within eachactivation-time interval. In the example of FIG. 20, the multiple sourcearray 2006 comprises six sources denoted by s₁, s₂, s₃, s₄, s₅, and s₆.As the survey vessel 2002 travels the sail line 2008 at a substantiallyconstant rate of speed, one or more of the sources of the multiplesource array 2006 are activated within the activation-time intervals.For example, open circle 2016 represents a time when source s₁ isactivated in the activation-time interval 2012. The sources may beactivated in the same order in each activation-time interval.Alternatively, the order in which the sources are activated may bechanged in different activation-time intervals. For example, the sourcesare activated in a different order in the activation-time intervals 2012and 2014. Two or more of the sources may be activated at approximatelythe same time as represented in activation-time interval 2014, in whichsources s₂ and s₅ are activated at approximately the same time. Thesources may be activated at randomly selected times in eachactivation-time interval, or the sources may be activated according to aparticular preselected order in each activation-time interval. Thenumber of sources activated in an activation-time interval may berandomly selected. In FIG. 20A, the activation-time intervals 2012-2014correspond to activation spatial intervals 2018-2020 along the sail line2008. After each activation of one or more sources in the multiplesource array 2006, pressure and/or particle motion wavefields aremeasured and seismic data recorded in shot records over longer durationrecording time intervals, such as time intervals 2022 and 2024, whichcorresponds to recording spatial intervals 2026 and 2028, respectively.The beginning of each recording time interval may coincide with thestart time of each activation-time interval. For example, recording timeinterval 2022 begins at t₀.

In alternative implementations, blended seismic data may be continuouslyrecorded along a sail line and activation of sources of a multiplesource array are not restricted to activation-time intervals. Activationof the sources may be separated by time intervals that are short enoughthat the source wavefields overlap and the continuously recorded seismicdata is blended continuously recorded seismic data. The sources of amultiple source array may be activated according to a sequence oractivated at random. The time intervals between activation of thesources of a multiple source array may be may be regular time intervalsor random time intervals.

FIG. 20B shows examples in which activation of the sources of themultiple source array 2006 are separated by time intervals while blendedseismic data is continuously recorded as the survey vessel travels thesail line 2008. Time axis 2030 shows one example in which the sixsources s₁, s₂, s₃, s₄, s₅, and s₆ of the multiple source array 2006 areactivated in a particular sequence separated by regularly spaced timeintervals. Time axis 2032 shows an example in which the six sources s₁,s₂, s₃, s₄, s₅, and s₆ of the multiple source array 2006 are activatedat random and at randomly spaced time intervals. In alternativeimplementations, the sources of a multiple source array may be activatedin a sequential order but at randomly spaced time intervals. In stillother implementations, the sources of a multiple source array may beactivated at random and separated by regularly spaced time intervals.

The seismic data recorded in a recording time interval is blendedseismic data when source and reflected wavefields created by activationsof two or more sources of a multiple source array in the samesource-activation-time interval overlap. In other words, a shot recordthat records overlapping source and reflected wavefields created byactivations of two or more sources in the same source-activation-timeinterval comprises blended seismic data.

The difference between blended seismic data and unblended seismic datamay be distinguished as follows. For unblended seismic data, the seismicdata matrix P for a single frequency fin the seismic data set Pdescribed above with reference FIG. 8, may be represented byP=DXS  (15)

where

D is a receiver matrix;

X represents an impulse response matrix of the subterranean formation;and

S represents a source matrix.

The source matrix S is an N×N diagonal matrix in which each diagonalelement is a far-field signature of one of the sources in the multiplesource array having a source location

_(s). The far-field signatures may be determined by near-field pressuremeasurements of the source elements comprising each of the sources. Thereceiver matrix D is an M×M matrix in which each matrix elementrepresents receiver sensitivity for each frequency and direction. Theelements of D convert the seismic wavefield measured by a receiver at areceiver location

_(r) into a signal generated by the receiver. The impulse responsematrix X is an M×N matrix in which each element represents how animpulse of the subterranean formation to a source wavefield generated bya source at the source location

_(s) and a signal generated at the receiver location

_(r). The seismic data matrix P at a frequency f is given by:

$\begin{matrix}{{P\left( {{\overset{\rightharpoonup}{x}}_{r},{\overset{\rightharpoonup}{x}}_{s},f} \right)} = {\sum\limits_{r = 1}^{M}\;{\sum\limits_{s = 1}^{N}\;{{d\left( {\overset{\rightharpoonup}{x}}_{r} \right)}{{ir}\left( {{\overset{\rightharpoonup}{x}}_{r},{\overset{\rightharpoonup}{x}}_{s}} \right)}{{ffs}\left( {\overset{\rightharpoonup}{x}}_{s} \right)}}}}} & (16)\end{matrix}$

where

d(

_(r)) is a diagonal element of receiver matrix D;

ir(

_(r),

_(s)) is a matrix element of impulse response matrix X; and

ffs(

_(s)) is a far-field signature of the source matrix S.

In practice, the unknown impulse response X is obtained from the seismicdata matrix P by removing the source matrix S (i.e., designaturing) andthe receiver matrix D.

On the other hand, with blended seismic data, traces in the seismic dataset described above with reference to FIG. 7 represent blended sourceand receiver wavefields associated with different source activations.The seismic data set comprises traces of blended seismic data and isdenoted by p _(bl). The blended seismic data set p _(bl) is transformedfrom the s-t domain to the s-f domain to obtain a blended seismic dataset, p _(bl), in the s-f domain. Blending may be represented in the s-fdomain by introducing a source blending matrix Γ that transforms thesource matrix S into a blended source matrix:S _(bl) =SΓ  (17)The source blending matrix Γ models the physical interaction ofoverlapping source wavefields. Matrix elements of the blending matrix Γare represented byγ_(sj) =B _(sj) e ^(−iωΔt) ^(sj)   (18)

where

s is a source index;

j is an activation-time interval index;

B_(sj) is relative amplitude; and

Δt_(sj) is relative time delay with respect to a first source fired inthe jth activation-time interval.

If the sth source is not activated in the jth activation-time interval,then the associated blending matrix element in Equation (18) is zero. Onthe other hand, if the sth source is the first source activated in thejth activation-time interval, then Δt_(sj)=0 and the associated blendingmatrix element in Equation (18) is one. If the sth source is activatedafter a first source is activated in the jth activation-time interval,relative time delay Δt_(sj) between activation of the first and the sthsource is not zero in Equation (18). The blending matrix superimposesfar-field signatures of specific sources in the multiple source array.The receiver matrix D is not affected when the sources are activated tocreate blended seismic data, and the impulse response matrix X isindependent of the acquisition design. As a result, a blended seismicdata matrix in the blended seismic data set P _(bl) for a particularfrequency f is given by:P _(bl) =DXS _(bl)=DXST=PΓ  (19)

Before the source matrix S and the receiver matrix D can be removed fromthe blended seismic data matrix in Equation (19), the blending matrix Γis removed in a process called deblending. Because Equation (19) istypically underdetermined, the seismic data matrix P cannot be directlysolved for by P=P_(bl)Γ⁻¹. Instead, deblending may be performed using aniterative process shown in FIGS. 21 and 22.

FIG. 21 shows an example flow-control diagram that implements a methodto deblend blended seismic data. A loop beginning with block 2101repeats the operations represented by blocks 2102-2111 for eachactivation of one or more sources of a multiple source array insource-array activation-time intervals, as described above withreference to FIG. 20. In block 2102, a blended seismic data set p _(bl)is transformed from the s-t domain to the s-f domain to obtain a blendedseismic data set P _(bl). In block 2103, a blending matrix Γ is formedfrom the recorded activation times of the sources activated in thesource-array activation-time interval. A conjugate transpose of theblending matrix Γ gives conjugate transposed blending matrix, Γ^(H),where the superscription H represents conjugate transpose. A loopbeginning with block 2104 repeats the operations represented by blocks2105-2108 for each frequency of the blended seismic data set P _(bl).Each frequency of the blended seismic data set P _(bl) corresponds to ablended seismic data matrix P_(bl), as described above with referenceFIGS. 8 and 9. In block 2105, a pseudo-deblended seismic data matrix iscomputed as follows:P _(ps) =P _(bl)Γ^(H)  (20)In block 2106, the pseudo-deblended seismic data matrix P_(ps) istransformed from the s-f domain to the w-f frequency domain as describedabove with reference to FIG. 12 and filtering is performed in the w-fdomain to attenuate noise as described above with reference to FIG.13A-13C. After filtering in the w-f domain, the filteredpseudo-deblended seismic data is transformed back to the s-f domain. Inblock 2107, a routine “estimate deblended seismic data” is called tocalculate an estimated deblended seismic data matrix, P_(deb), from thepseudo-deblended seismic data matrix P_(ps). In decision block 2108, theoperations represented by blocks 2105-2107 are repeated for a blendedseismic data matrix associate with a different frequency of the blendedseismic data set P _(bl). In decision block 2109, the operationsrepresented by blocks 2102-2108 are repeated for another activation ofthe sources.

The deblended seismic data may be collected and used to calculate imagesof the subterranean formation. The images may reveal various structuresand layers within the subterranean formation.

FIG. 22 shows an example flow-control diagram that implements theroutine “estimate deblended seismic data” called in block 2107 of FIG.21. In block 2201, the pseudo-deblended seismic data matrix P_(ps) issorted into the common-receiver-station domain:P _(ps) ={P _(ps) ^(pr)}_(r=1) ^(M)  (21)

where P_(ps) ^(r) represents a pseudo-deblended common-receiver-stationgather of the pseudo-deblended seismic data matrix P_(ps) for the rthreceiver location.

A loop beginning with block 2202, repeats that operations of blocks2203-2211 for each common-receiver-station gather P_(ps) ^(r) in the set{P_(ps) ^(r)}_(r=1) ^(M). In block 2203, the pseudo-deblendedcommon-receiver-station gather P_(ps) ^(r) is used to initialize a firstestimated deblended common-receiver-station gather P₁ ^(r), and anestimated deblended seismic data matrix P_(deb) is initialized as theempty set. Blocks 2204-2209 represent an iterative process ofdetermining an estimated deblended common-receiver-station gather fromthe pseudo-deblended common-receiver-station gather P_(ps) ^(r). Inblock 2204, the common-receiver-station gather P₁ ^(r) is sourcedeghosted as described above with reference to FIGS. 16-17. An estimateddeblended common-receiver-station gather is denoted by P_(j) ^(r), whereiteration j is an index. In block 2205, muting is applied in the w-fdomain outside a frequency cone as described above with reference toFIGS. 16-17. In block 2206, thresholding is applied to the estimateddeblended common-receiver-station gather P_(j) ^(r) by muting seismicdata with amplitudes that are less than a threshold amplitude togenerate a thresholded estimate of the deblended common-receiver-stationgather P _(j) ^(r). In block 2206, blending noise is calculated from thenoise-free deblended common-receiver-station gather, blending matrix,and the conjugate transposed blending matrix as follows:N _(j) ^(r) =P _(j) ^(r)(ΓΓ^(H) −I)  (22)

where I represents the identify matrix.

In block 2208, a j+1^(st) estimated deblended common-receiver-stationgather P_(j+1) ^(r) is calculated as follows:P _(j+1) ^(r) =P _(ps) ^(r) −N _(j) ^(r)  (23)In decision block 2209, when the following condition is satisfied for athreshold T|P _(j+1) ^(r) −P _(j) ^(r) |<T  (24)the iterative process represented by blocks 2205-2208 is considered tohave converged at the j+1^(st) estimated deblendedcommon-receiver-station gather P_(j+1) ^(r). Otherwise, control flows toblock 2210 in which the index j is incremented and the computationaloperations represented by blocks 2204-2208 are repeated.

In block 2211, the j+1^(st) estimated deblended common-receiver-stationgather P_(j+1) ^(r) is added to the estimated deblended seismic datamatrix P_(deb):P _(deb) =P _(deb) +P _(j+1) ^(r)  (25)In other words, in block 2211, the estimated deblendedcommon-receiver-station gathers that satisfy the condition given byEquation (24) are collected to form an estimated deblended seismic datamatrix P_(deb) for the activation of the multiple source array. Indecision block 2012, the operations represented by blocks 2203-2211 arerepeated for another pseudo-deblended common-receiver-station gather. Inan alternative implementation, rather than repeating the operationsrepresented by blocks 2204-2209 until the estimated deblendedcommon-receiver-station gathers converge, the operations represented byblocks 2204-2208 may be repeated for a fixed number iterations.

FIGS. 23A-23C show source deblending applied to acommon-receiver-station gather of pseudo-deblended seismic data producedwith a source of a vertical-source array. FIG. 23A showspseudo-deblended seismic data generated by a source located 28.125 mbelow the free surface and its corresponding ghost source. Speckledregions 2302 and 2304 represent reflection events produced by anothersource in the vertical-source array. The seismic data shows reflectionevents that correspond to the actual source, such as event 2306. Theseismic data also shows reflection events that are recorded later intime and correspond to the ghost source, such as event 2308. FIG. 23Bshows the results of deblending and source deghosting described above toremove reflection events associated with the other source and the sourceghost. FIG. 23C shows a common-receiver-station gather of unblended datafor a single source activated at the depth of 28.125 meters below thefree surface. Comparison of the gathers in FIGS. 23B and 23C reveals theeffectiveness of deblending and source deghosting methods describedabove.

FIGS. 24A and 24B show the deblended and unblended seismic data shown inFIGS. 23B and 23C transformed into the w-f domain. The black region ineach figure represents a zero value. FIG. 24C shows the error betweenthe transformed deblended and unblended seismic data calculated as thedifference between the results shown in FIGS. 24A and 24B. Very fewnon-zero points are present near the center of FIG. 24C, which indicatesdeblended and unblended seismic data shown in FIGS. 23B and 23C are inclose agreement. FIG. 24D shows seismic data in the w-f domain for asource at a depth of 28.125 m and the corresponding source ghost.

Deblending results, such as those shown in FIGS. 23 and 24, may beimproved by decreasing the strength (i.e., amplitude) of the sourcewavefields with distance from the center of the multiple source array.In particular, transformations, such as the Fourier transformation, areimproved over a source axis of seismic data as the strength of thesource wavefields generated by sources located farther from the centerof the multiple source array are decreased. In order to decrease thestrength or amplitude of the source wavefields with distance from thecenter of a multiple source array, the sources of the multiple sourcearray may be activated such that the farther a source is from the centerof the multiple source array, the fewer number of the source elements ofthe source are activated. Alternatively, sources located near the centerof the multiple source array may be implemented with more sourceelements and/or with source elements having larger chamber volumes thansource elements of sources located farther away from the center of themultiple source array.

FIG. 25 shows example plots of source strength (i.e., amplitude) ofsources in a multiple source array that decrease with increasingdistance from a center of the multiple source array. Horizontal axis2502 represents distance from the center of the multiple source array.Vertical axis 2504 represents source strength (i.e., amplitude). Curve2506 represents source strength that exponentially decreases from thecenter of a multiple source array. Curve 2508 represents source strengththat linearly decreases from the center of a multiple source array.Curve 2510 represents source strength that parabolically decreases fromthe center of a multiple source array. Curves 2506, 2508, and 2510represent examples of ways in which the strength of the sourcewavefields generated by sources of a multiple source array may decreaseas the distance of the sources increase from the center of the multiplesource array.

FIG. 26 shows an example computer system that executes efficient methodsof attenuating noise, source deghosting, and deblending seismic data andtherefore represents a geophysical-analysis data-processing system. Theinternal components of many small, mid-sized, and large computer systemsas well as specialized processor-based storage systems can be describedwith respect to this generalized architecture, although each particularsystem may feature many additional components, subsystems, and similar,parallel systems with architectures similar to this generalizedarchitecture. The computer system contains one or multiple centralprocessing units (“CPUs”) 2602-2605, one or more electronic memories2608 interconnected with the CPUs by a CPU/memory-subsystem bus 2610 ormultiple busses, a first bridge 2612 that interconnects theCPU/memory-subsystem bus 2610 with additional busses 2614 and 2616, orother types of high-speed interconnection media, including multiple,high-speed serial interconnects. The busses or serial interconnections,in turn, connect the CPUs and memory with specialized processors, suchas a graphics processor 2618, and with one or more additional bridges2620, which are interconnected with high-speed serial links or withmultiple controllers 2622-2627, such as controller 2627, that provideaccess to various different types of non-transitory computer-readablemedia, such as computer-readable medium 2628, electronic displays, inputdevices, and other such components, subcomponents, and computationalresources. The electronic displays, including visual display screen,audio speakers, and other output interfaces, and the input devices,including mice, keyboards, touch screens, and other such inputinterfaces, together constitute input and output interfaces that allowthe computer system to interact with human users. Computer-readablemedium 2628 is a data-storage device, including electronic memory,optical or magnetic disk drive, USB drive, flash memory and other suchdata-storage device. The computer-readable medium 2628 can be used tostore machine-readable instructions that encode the computationalmethods described above and can be used to store encoded data, duringstore operations, and from which encoded data can be retrieved, duringread operations, by computer systems, data-storage systems, andperipheral devices.

The methods and systems described above produce seismic data generatedusing multiple source arrays. The seismic data may be deblended, sourcedeghosted, and undergo noise attenuation as described above. Theresulting seismic data may then be further processed to obtain accurate,high-resolution seismic images of subterranean formations locatedbeneath a body of water. High-resolution seismic images of asubterranean formation may, in turn, be used to determine the structureof a subterranean formation. The structural information may revealpetroleum reservoirs and may be used to monitor petroleum reservoirsduring production.

The methods and systems disclosed herein may be use to form or process ageophysical data product indicative of certain properties of asubterranean formation. The geophysical data product may includegeophysical data such as pressure data, particle motion data, particlevelocity data, particle acceleration data, and any seismic image thatresults from using the methods and systems described above. Thegeophysical data product may be stored on a non-transitorycomputer-readable medium as described above. The geophysical dataproduct may be produced offshore (i.e., by equipment on the surveyvessel 102) or onshore (i.e., at a computing facility on land).

It is appreciated that the previous description of the disclosedembodiments is provided to enable any person skilled in the art to makeor use the present disclosure. Various modifications to theseembodiments will be readily apparent to those skilled in the art, andthe generic principles defined herein may be applied to otherembodiments without departing from the spirit or scope of thedisclosure. Thus, the present disclosure is not intended to be limitedto the embodiments shown herein but will be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

The invention claimed is:
 1. A method of collecting seismic data in amarine survey, the method comprising: towing a cross-line source arrayby a survey vessel, the cross-line source array having three or moresources in a linear arrangement, wherein a total length of thecross-line source array in the cross-line direction is less than aseparation distance between any two adjacent streamers towed by thesurvey vessel, and a separation distance between adjacent sources in thecross-line direction is about the distance between adjacent streamersdivided by the number of sources in the cross-line source array;activating the three or more sources of the cross-line source array,each source producing a source wavefield; and recording seismic datagenerated by receivers located along the streamers in a non-transitorycomputer-readable medium, each receiver measuring source wavefieldscreated by the cross-line source array and reflected wavefieldsgenerated by a subterranean formation in response to the sourcewavefields.
 2. The method of claim 1, wherein activating the sources ofthe cross-line source array comprises activating each of the sources atrandomly selected times in an activation time interval such that thesource wavefields overlap, the reflected wavefields overlap, and theseismic data is blended seismic data.
 3. The method of claim 1, whereinactivating the sources of the cross-line source array comprisesactivating the sources according to a sequence in an activation timeinterval such that the source wavefields overlap, the reflectedwavefields overlap, and the seismic data is blended seismic data.
 4. Themethod of claim 1, wherein an amplitude of each source wavefielddecreases with increasing distance from a center of the cross-linesource array.
 5. The method of claim 1, wherein activating sources ofthe cross-line source array comprises activating fewer source elementsper source of the cross-line source array with increasing distance froma center of the cross-line source array.
 6. The method of claim 1,comprising calculating cross-line particle velocities at the receiverlocations from the seismic data.
 7. The method of claim 6, wherein theseismic data includes pressure data and calculating the cross-lineparticle velocity comprises: calculating an approximate cross-lineparticle velocity at a point between two locations of two sources of thecross-line source array from the pressure data generated by a pressuresensor at the receiver location for activation of a first of the twosources and pressure data generated by the pressure sensor at thereceiver location for activation of a second of the two sources;applying source location reciprocity to the approximate cross-lineparticle motion velocity at the point between the two sources togenerate an approximate cross-line particle velocity at the receiverlocation.
 8. The method of claim 7, wherein calculating the approximatecross-line particle motion velocity at the point between the two sourcesfurther comprises: transforming the pressure data recorded foractivation of the two sources from the space-time domain to thespace-frequency domain; calculating a pressure gradient at the pointbetween the two sources from the pressure data associated withactivation of the first of the two sources and the pressure dataassociated with activation of the second of the two sources; andmultiplying the pressure gradient by the cross-line particledisplacement direction to generate a pressure gradient in the cross-linedirection; dividing the pressure gradient in the cross-line direction bya product of an angular frequency and speed of sound in water; andapplying source location reciprocity to the pressure gradient in thecross-line direction to generate the approximate cross-line particlemotion velocity at the receiver location.
 9. The method of claim 6,further comprising interpolating cross-line particle velocities betweenreceivers of adjacent streamers to reduce cross-line spatial aliasingbased on the approximate cross-line particle velocity at the receiverlocations.
 10. A system to collect seismic data generated with across-line source array, the system comprising: a cross-line sourcearray having three of more linearly arranged sources in the cross-linedirection, each source having a number of source elements, wherein atotal length of the cross-line source array in the cross-line directionis less than a separation distance between any two adjacent streamerstowed by the survey vessel, and a separation distance between adjacentsources in the cross-line direction is about the distance betweenadjacent streamers divided by the number of sources in the cross-linesource array; a seismic source control that controls activation of thesources, each source produces a source wavefield; and recordingequipment that records seismic data generated by one or more receiversthat measure source wavefields created by the cross-line source arrayand reflected wavefields generated by a subterranean formation inresponse to the source wavefields.
 11. The system of claim 10, whereinnumber of source elements per source of the cross-line source arraydecreases with increasing distance from a center of the cross-linesource array.
 12. The system of claim 10, wherein chamber volumes ofsource elements per source of the cross-line source array decreases withincreasing distance from a center of the cross-line source array.
 13. Amethod of manufacturing a geophysical data product, the methodcomprising: towing a cross-line source array by a survey vessel, thecross-line source array having three or more sources in a lineararrangement, wherein a total length of the cross-line source array inthe cross-line direction is less than a separation distance between anytwo adjacent streamers towed by the survey vessel, and a separationdistance between adjacent sources in the cross-line direction is aboutthe distance between adjacent streamers divided by the number of sourcesin the cross-line source array; activating the three or more sources ofthe cross-line source array, each source producing a source wavefield;and recording seismic data generated by receivers located alongstreamers in a non-transitory computer-readable medium, each receivermeasuring source wavefields created by the cross-line source array andreflected wavefields generated by a subterranean formation in responseto the source wavefields.